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Séminaire de H.N.W. Lekkerkerker

Gelation versus Liquid Crystal Phase Transitions in Suspensions of Charged Mineral Colloidal Platelets

H.N.W. Lekkerkerker,

Van ’t Hoff Laboratory for Physical and Colloid Chemistry,Debye Institute Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

Gelation versus Liquid Crystal Phase Transitions in Suspensions of Charged Mineral Colloidal Platelets

One of the most remarkable phenomena exhibited by concentrated suspensions of colloidal particles is the spontaneous transition from fluid-like structures to those exhibiting long-range spatial and/or orientational order (colloidal crystals and colloidal liquid crystals). The fact that such ordering can occur in suspensions in which interparticle forces are purely repulsive provides a dramatic realization of the predictions made by Lars Onsager in the 1940’s and later substantiated by computer simulations. From these studies it is clear, that the ordering is driven by entropy.
Liquid crystalline phases in suspensions of rod like mineral colloidal particles have been known for a long time. As early as 1925 Zocher reported on the observation of a nematic phase in suspensions of V2O5 and a smectic phase in suspensions of FeOOH. A decade later Langmuir reported on sols of platelike clay particles that after standing for several 100 hours separated into an isotropic and nematic phase. In the last decade ,after a period of 60 years with no new experimental results, interesting liquid crystal phase transitions have been observed in suspensions of both synthetic as well as natural (clay) plate like particles After a brief review of some of these results I will focus on the equilibrium and non-equilibrium phases of suspensions of charged gibbsite platelets. We study the competition between liquid crystal formation and gelation at different colloid concentrations and salt strengths. By variation of the ionic strength we are able to tune the effective thickness-to-diameter ratio of the platelets in suspension and the balance between attractive and repulsive interactions. We find an isotropic (I) nematic (N) and columnar (C) phase wedged between an attractive gel and a repulsive gel (glass). A model for the repulsive gel is proposed.Observations on the devitrification of this glass phase and recent computer simulations confirm this model Results of liquid crystal phases of gibbsite platelets as templates for the generation of ordered silica structures and polymer nanocomposites are presented